The present embodiments relate to timing measurement in positron emission tomography (PET). Blocks of detectors detect gamma rays emitted indirectly by a positron-emitting tracer. Using spatially diverse detectors, pairs of gamma rays generated by a same positron may be detected. The pairs of gamma rays travel about 180 degrees apart. To distinguish specific pairs, the coincidence of detected gamma rays is determined. The timing of receipt is used to pair the detected gamma rays, detecting events along lines of response. Time of flight may be used to detect segments of the line of response.
The timing of a detected event is determined using a timing circuit. For example, a time-to-digital converter is used to determine a time at which the event occurred. Different types of timing circuits have been proposed. Different advantages and weaknesses are provided by each. For example, constant fraction discrimination is accurate, providing sufficient accuracy for time of flight to select a segment of the line of response. However, constant fraction discrimination is susceptible to errors due to scattering and may not detect a time for subsequent events during a pile up. As another example, second derivative discrimination may detect subsequent events during a pile up, but has less timing accuracy or resolution than constant fraction discrimination.